The bandwidth of a National Television Standards Committee (NTSC) television channel is six megahertz (MHz), and is unlikely to change in the near future. Recently, proposals have been made for High Definition Television (HDTV). An HDTV picture has approximately twice the resolution in both the horizontal and vertical directions as an NTSC television picture. Therefore, in order to transmit the HDTV picture, a larger information bandwidth is required than the 6 MHz provided by the NTSC television channel.
Several systems have been proposed for transmitting the HDTV picture using only the 6 MHz bandwidth provided by the NTSC channel. One such system is described in U.S. Pat. No. 4,905,084, issued to L. Zucker on Feb. 27, 1990. The Zucker patent describes a system whereby two signals are transmitted on the same NTSC television channel by employing two carriers which are polarized orthogonally to each other. The first signal is an NTSC signal, while the second signal is the additional data required to upgrade the NTSC television picture to a higher resolution television picture, although the higher resolution television picture has somewhat less resolution than an HDTV picture.
Other proposals have suggested using two adjacent NTSC channels to transmit the HDTV picture, or using upper and lower sideband modulation on a single channel. While all of these proposals provide a somewhat acceptable picture, it can be appreciated that there is always some interference caused by the additional information being transmitted on the same channel as the NTSC signal. Consequently, the NTSC picture, when received by an NTSC television receiver, appears somewhat degraded from the way it would appear if it were transmitted without any additional information on the channel.